Interest in energy storage technologies has been growing in recent years. As the fields of application have expanded to the energy of mobile phones, camcorders, and laptops, and further to electric vehicles, efforts for the research and development on electrochemical devices have increasingly materialized. The field of electrochemical devices is receiving the most attention, and among these, the development of chargeable and dischargeable secondary batteries has become the focus of attention.
Among secondary batteries currently used, lithium secondary batteries developed in the early 1990s are highly favored due to the advantages of high operating voltage and high energy density compared to conventional batteries such as Ni-MH. However, lithium secondary batteries have a potential danger of causing an explosion due to the generation of an exothermic reaction depending on the environment of use. Particularly, a polyolefin-based porous substrate that is commonly used as a separator of an electrochemical device has a problem in that it shows a severe thermal shrinking behavior at a temperature of 100° C. or higher due to material characteristics and manufacturing process characteristics that include elongation, thereby causing a short circuit between a cathode and an anode.
In order to solve this safety problem of electrochemical devices, Korean Patent Application Laid-Open Publication Nos. 10-2006-72065, 10-2007-231, 10-2008-97364, and the like have proposed a separator in which an inorganic material coating layer made of a mixture of inorganic particles and a binder polymer is formed on at least one surface of a porous polymer substrate having multiple pores. The inorganic particles in the inorganic material coating layer formed on the porous polymer substrate play a role of a kind of a spacer that maintains the physical form of the coating layer, and prevent a cathode and an anode from being in direct contact with each other even when the porous polymer substrate is damaged.
As described above, the inorganic material coating layer formed on the surface of the porous polymer substrate can contribute to the thermal safety enhancement of an electrochemical device, therefore, the development of separators capable of further enhancing the heat resistance of an electrochemical device by adopting a binder polymer having different chemical structures from those proposed in existing patents has continued. In addition, there have been demands for the development of separators that can improve high temperature cycle performance and discharge characteristics of an electrochemical device.
Accordingly, a method for enhancing the heat resistance of a separator by forming a ceramic/binder porous coating layer in a polyolefin-based separator having weak heat resistance using polyvinylidene fluoride-hexafluoropropylene as a binder has been proposed. However, the binder used therein has a melting point of approximately 150° C. and thereby has a limit in heat resistance.
In addition, Korean Patent Application Laid-Open Publication No. 10-2008-97364 uses a binder having a cross-linking structure, however, a thermal initiator such as AIBN that produces radicals when decomposed by heat is used instead of a common chemical curing agent, therefore, the radicals generated therefrom attack the main chain of the binder polymer and as a result, the main chain of the polymer is cross-linked. Therefore, the enhancement in heat resistance has a limit in a method such as this, and in the results, the change in thermal shrinkage percentage, which is the most clear evidence of heat resistance enhancement, have not been described.
In view of the above, the inventors have developed a separator containing a porous polymer substrate and an inorganic particle layer formed on at least one surface of the porous polymer substrate, wherein a binder connects between the inorganic particles, and between the inorganic particles and the porous polymer substrate, the binder is formed on the porous polymer substrate by curing polyester that includes one or more hydroxyl groups, carboxyl groups or both at the end, in the side chain or both using an isocyanate-based curing agent at a temperature at which the porous polymer substrate does not shrink by 10% or more in length, the porous polymer substrate and the network-structured binder formed by the curing reaction of the polyester and the curing agent are integrated, and the inorganic particles are collected and fixed inside the network-structured binder, and have verified that, when the separator is used in an electrochemical device, a short circuit between a cathode and an anode can be suppressed even when the electrochemical device is overheated, since the thermal shrinkage of the porous polymer substrate that is used as a separator is suppressed when the electrochemical device is overheated, and that effects enhancing the high temperature cycle characteristics of the electrochemical device can be obtained, and have thereby completed the present invention.